# performance.now + Force heavy task
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**Exploit taken from [https://blog.huli.tw/2022/06/14/en/justctf-2022-xsleak-writeup/](https://blog.huli.tw/2022/06/14/en/justctf-2022-xsleak-writeup/)**
In this challenge the user could sent thousands of chars and if the flag was contained, the chars would be sent back to the bot. So putting a big amount of chars the attacker could measure if the flag was containing in the sent string or not.
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Initially, I didn’t set object width and height, but later on, I found that it’s important because the default size is too small to make a difference in the load time.
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{% hint style="success" %}
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